FIELD OF THE INVENTION
The invention relates to a method for determining a fraction of a gas mixture in a containment atmosphere of a nuclear power station by measuring a temperature change resulting from a catalytic reaction. The invention further relates to a device for implementing the method.
It is known that hydrogen in the presence of a catalyst, for instance based on platinum or palladium, is oxidized in an exothermal reaction even at room temperature. That catalytic oxidation of hydrogen is also known as cold combustion. The hydrogen concentration in a gas mixture can be determined through the use of a measurement of the heat of reaction or temperature change resulting from the reaction.
Thus, for example, in a method disclosed by U.S. Pat. No. 4,298,574, the temperature change arising as a result of a catalytic oxidation of hydrogen on a catalyst is detected, with respect to a reference value, by means of a thermoelement, and is converted into a corresponding voltage signal. The measured voltage is a measure for the hydrogen fraction of the gas mixture. That method can also be used to determine the fraction of carbon monoxide or hydrocarbon in the gas mixture.
In a method disclosed by German Published, Non-Prosecuted Application DE 30 46 560 A1 for detecting flammable gases, especially hydrogen in the containment atmosphere of a nuclear power station, a temperature change is detected by means of a temperature-dependent ohmic resistor which forms part of a bridge circuit.
In order to ensure that a reactive hydrogen/oxygen mixture for the detection of hydrogen in an inert gas stream is obtained, a method disclosed by U.S. Pat. No. 4,072,043 involves the admixture of air to the inert gas (e.g. N2) prior to its entry into a detection chamber.
The known methods do pose the risk of the catalyst heating up as far as the ignition limit of an ignitable mixture. Due to the high heat release that may result in destruction of the catalyst. In the containment atmosphere of a nuclear power station, ignitable mixtures can react at high rates in an uncontrolled manner even with a hydrogen fraction of less than 10%, and reliable measurement of the fractions of the mixture are then no longer possible.
In order to enable satisfactory detection of a flammable gas, even if the concentration of the flammable gas in a gas sample increases beyond a limit value, Published UK Application GB 2 153 073 A, corresponding to U.S. Pat. No. 4,565,086, discloses pumping a diluent, especially ambient air, into a line carrying the gas sample.